In many instances, it is desirable to detect and identify a power line. For example, it is desirable for aircraft, such as helicopters and airplanes, to detect and avoid power lines. Thus, detectors have been developed and employed on aircraft to detect power lines such that the pilot of the aircraft is alerted if the aircraft is approaching a power line and can make appropriate evasive navigational actions.
One sensor for detecting power lines is illustrated in U.S. Pat. No. 4,277,771 to Lawrey H. Chapin, et al., which issued on Jul. 7, 1981 (the '771 patent). The '771 patent discloses an electric induction field navigation apparatus for detecting power lines and assisting a pilot in guiding an aircraft. The electric induction field navigation apparatus of the '771 patent includes a plurality of field gradient detectors. Each field gradient detector is a parallel plate capacitor which produces a voltage having an amplitude dependent on the gradient of the electric field to which the detector is exposed. Thus, the electric field produced by high voltage power transmission lines can be detected and the aircraft can be guided appropriately.
Another sensor for detecting power lines is disclosed in U.S. Pat. No. 4,649,375 to Denis E. Duppong, et al., which issued on Mar. 10, 1987 (the '375 patent). The sensor of the '375 patent is used in conjunction with construction equipment, such as cranes, which work in the vicinity of electrical power lines. The sensor includes a pickup coil having an elongated laminated core formed of a ferromagnetic material, such as iron, and around which a multi-turn coil of wire is wound. Thus, the magnetic field produced by the current conducted by an electric power line induces a current in the multi-turn coil. The output of the pickup coil, i.e., the induced current, is coupled through an amplifier detector module and is compared to a reference signal to determine if the sensor and, thus, the crane is in the vicinity of a power line.
The sensor of the '375 patent does not, however, determine if the sensor is approaching the detected power line, i.e., if the crane is moving toward the detected power line. Instead, the sensor of the '375 patent detects power lines in its vicinity including, not only the power lines which the sensor, and thus the crane, is approaching, but also power lines which the sensor is not approaching and are therefore not a threat. Thus, the operator of the crane is identified of all power lines in its vicinity including those which the crane is not approaching and for which the operator need not adjust the position of the crane.
A pickup coil having a ferromagnetic core, such as the ferromagnetic core of the sensor of the '375 patent, increases the sensitivity of a sensor in comparison to a sensor having a pickup coil which does not include a ferromagnetic core. However, a ferromagnetic core conducts eddy currents which are also induced within the core by the magnetic field produced by the current conducted by the power lines. The induced eddy currents contribute to the loss of the sensor and, therefore, increase the resistance of the sensor and correspondingly decrease the sensitivity of the sensor.
As known to those skilled in the art, sensors which have a decreased sensitivity detect, not only signals of a desired frequency or within a desired range of frequencies, but also signals within a wider range of frequencies, including extraneous electromagnetic interference or noise having a frequency outside the desired range of frequencies. Likewise, the increased resistance of a sensor will decrease the resulting voltage or output signal of the sensor such that differences between signals produced by the magnetic field generated by power lines and signals produced by extraneous electromagnetic noise will be more difficult to ascertain. Thus, the range or the distance from the power lines at which a sensor is able to detect power lines is also limited by the decreased sensitivity of the sensor.